High voltage switching transistors, such as power MOSFETs, junction field effect transistor (JFETs) and gallium nitride (GaN) high electron mobility transistor (HEMT), are commonly used as semiconductor switches in high voltage and high power devices such as switched-mode power supplies, motor controllers, and high voltage and high power switching circuits. Some of these devices, such as the GaN HEMT have the ability to be operated at very high voltages without the device breaking down or becoming damaged.
Some devices, such as the JFET and the GaN HEMT may be fabricated to have a negative threshold voltage, thereby causing the device to be conductive when zero voltage is across the gate and source of these transistors. Such devices are accordingly referred to as “normally-on” devices or transistors, since the devices are effectively on under zero bias conditions. When using such normally-on devices, provisions are generally made to ensure that a voltage is generated to make sure that the normally-on device may be turned off. For example, in a driver circuit used in a switch mode power supply, a negative voltage is generated or provided that has a voltage that is sufficiently below the threshold of the normally-on device to ensure that the device is in fact turned off as intended.
Another issue pertaining to using normally-on devices is the possibility of high current transients during the startup of a system in which normally-on devices are used. For example, if a normally-on device is coupled between the terminals of a high voltage power supply, large currents may result when power is applied to the system. In order to avoid these high startup currents, a normally-off device, such as an enhancement-mode MOSFET may be coupled in series with the normally-on device. Once the power supplies that are configured to provide the turn-off voltage to the normally-on device have attained a voltage sufficient to turn off the normally-on device, then the normally-off device may be turned on.
In some cases, however, the sequencing of the system during power up utilizes various circuits to control the timing and behavior of the system as power is being applied. In such cases, an auxiliary power supply may be used to provide power to the startup circuitry.